Faraday’s Experiments

Section 38.1 Faraday’s Experiments

In April of 1820 Hans Christian Øersted of Denmark discovered that magnetic needles respond to electric currents. Here is a web link to a demo

of that effect, where you will see a magnetic needle deflect when current passes through a metallic rod.

The discovery by Øersted inspired many other scientists, including French physicists, Jean-Baptiste Biot, Félix Savart, and André-Marie Ampère. In the same year Marie Ampere in France found that magnets exert force on current-carrying wires. Ampere also demonstrated that wires carrying currents exerted force on each other.

These findings brought great excitement to the scientific community in 1820s since they provided definite experimental evidence of the close connections between electricity and magnetism - the two fields that had been hitherto separate and distinct from each other.

Soon people began to wonder whether the reverse was also true, i.e., a magnetic field would produce an electric current as well. Most of experiments to test this reverse process didn’t work out. For instance, strong magnets next to a wire failed to produce any detectable current in the wire. Even the magnetic field produced by large steady currents in a wire failed to cause any current in a loop of wire next to the first wire. We call these experiments null experiments since the result is zero. They are illustrated in Figure 38.1. The key was using the changing magnetic field as we will see below.

Figure 38.1. The Null Experiments. (a) When a loop of conducting wire and a magnet are fixed in their places no current is observed in the loop regardless of the strength of the magnet. (b) When a loop of conducing wire is near a current carrying wire, no current is observed in the loop if the current in the current-carrying wire is steady or if there is no relative motion between the two wires.

Moving Magnet Near a Metallic Loop

Around 1831 Michael Faraday found the missing link in these studies. He discovered that the electric effect of a magnetic field required that something must be changing with time. He observed that during the time a magnet is moving near a loop of wire a current is detected in the wire, but when the magnet stopped moving, there was no current in the wire as illustrated in Figure 38.2. Faraday understood this effect using his idea of magnetic field and magnetic flux.

Figure 38.2. *Moving magnet causes an induced current.* (a) When a magnet is moving towards a loop of conducting wire, magnetic flux is seen to be increasing during the time bar is moving towards the loop of wire. The changing magnetic flux through the loop causes an induced current in the loop of wire as detected by the galvanometer. (b) When the magnet is moving away, induced current has the opposite direction, due to decreasing flux through the loop. When there is no relative motion of the magnet and the loop no current is induced. Only during the time there is motion, there is induced current.

The direction of induced current is more easily seen by an application of Lenz’s law. Lenz’s law says induced current is in such as direction that its magnetic field opposes change in magnetic field through the loop occuring by whatever is changing in that loop’s environment. We will practice Lenz’s law later in the chapter. Here, you should see able to verify the induced current directions in each picture.

Moving a Metallic Loop Near a Magnet

Similarly, as illustrated in Figure 38.3, if a loop of wire is moving, e.g., if you are shaking around a loop of wire, in a space where there is some nonzero magnetic field, then a current will be detected in the wire during the time when the wire is moving but no there would be no current when you stop the motion of the wire.

Figure 38.3. *Moving loop in a magnetic field induces a current.* (a) When a loop of wire of conducting wire is moving in a space where there is a magnetic field, the loop develops an induced current as detected by the galvanometer. (b) When the loop is moving in the opposite direction the induced current has the opposite direction. When there is no relative motion of the magnet and the loop no current is induced. Only during the time there is motion, there is induced current.

Changing Current in a Circuit near a Metallic Loop

Since magnetic field is also present near a current carrying wire, you can replace magnet in the above experiments and work with two wires, through one of which you pass some current by connecting it to a power source. When current in the wire with the power source is changing, then, simultaneously a current is detected in the other wire even though there is no apparent current source in the second wire. But when current in the first wire is not changing, then there is no current in the second wire as illustrated in Figure 38.4. In all these situations, we say that currents are induced whenever magnetic field through the loop, i.e., magnetic flux, is changing.

Figure 38.4. *Changing current in one loop creates an induced current in another loop.* When current in loop 1 is changing, an induced current is observed in the loop 2. (a) When switch in circuit 1 is closed, current increases from zero to a non-zero value. During this time of increasing current in loop 1, the galvanometer shows that there is an induced current in loop 2. (b) When switch in circuit 1 is opened, current drops from a non-zero value to zero. During this time of decreasing current in loop 1, galvanometer shows that there is an induced current in loop 2, which is in the opposite direction to the induced current when current in 1 was increasing. Not shown: when current is loop 1 is steady, no induced current occurs in loop 2 regardless of the value of the current in loop 1.

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